Analog-to-digital converters (A/D converters) are used to convert analog input signals into digital output signals so that the digital signals may be further processed in the digital domain. The digital signals have a sample rate (typically expressed in samples per second), wherein for each time increment the analog signal will be sampled and quantized into a digital value having a specified number of bits. For example, music on a compact disc is generally sampled at a rate of 44.1 kHz and quantized to values represented by 16 bits. A/D conversion, however, may be performed at any sampling rate and the digital values may be represented by any number of bits.
One example of an A/D converter includes a flash A/D converter. A flash A/D converter may operate by comparing an input signal to a set of reference voltage thresholds. The reference voltage thresholds may be created by applying a high voltage to one end of a series of resistors, applying a low voltage to the other end of the series of resistors, and tapping the connections between each adjacent pair of resistors to serve as a reference voltage threshold. For each of the reference voltage thresholds, a comparator may be used to compare the analog input signal to the reference voltage threshold, and the comparator may output a one when the analog input signal has a voltage greater than or equal to the corresponding reference voltage threshold, and a zero otherwise.
The output of the comparators is sometimes referred to as a thermometer code. The term “thermometer code” is descriptive of the output in that the “ones” move “up” the comparators as the voltage value of the analog input signal increases in a way that resembles the mercury moving up a thermometer as the temperature increases. The thermometer code may be used directly or may be converted into a digital signal with a specified number of bits to subsequently produce the digital representation of the analog input signal. A thermometer code may also be used to generate an analog signal. For example, the values of a thermometer code may be input into conversion elements that output a predetermined voltage (e.g., via voltage sources) depending on whether the associated value of the thermometer code is a one or a zero.